This invention relates to a method for monitoring neutron fluence and, more particularly, to a highly accurate method for measuring reaction rates induced by neutron exposure.
Federal government regulations require that nuclear reactor coolant pressure boundaries have sufficient margin to ensure that the boundary behaves in a non-brittle manner when stressed under operating, maintenance, testing and postulated accident conditions, and that the probability of rapidly propagating fracture is minimized. These requirements necessitate prediction of the amount of radiation damage to the reactor vessel throughout its service life, which in turn requires that the neutron exposure to the pressure vessel be monitored.
Methods currently used for such monitoring, known generally as high fluence neutron dosimetry, include helium accumulation fluence monitors, solid state track recorders, and radiometric monitors. In each of these methods, a neutron-induced reaction rate is measured. In the case of helium accumulation fluence monitors, integral (n, .alpha.) rates are measured by isotope dilution helium mass spectrometry. In the solid state track recorder method, neutron-induced integral fission rates are measured by counting fission tracks. In the case of radiometric monitors, neutron-induced reaction rates are measured by radiometric counting of activation products.
When these high fluence neutron dosimetry methods are applied at a nuclear power reactor, high measurement accuracy is a goal with 3-5% uncertainty or better being a general requirement. These conventional high fluence neutron dosimetry methods, however, have inherent accuracy limitations due to uncertainties in dosimeter mass and detection of product isotopes.
In light thereof, a simpler and more accurate high fluence neutron dosimetry method is needed for measuring neutron exposure within a nuclear power reactor.